Welcome to the forums,
That's anything but a standard PSU form factor. Contact Dell either way, but if you want to continue to use that case ask about a replacement. Otherwise, ask if you can buy a new case/PSU and transfer stuff over. Read something that they have a 10pin CPU connector. You won't have an I/O shield for the back panel and might need a little custom modding to get the dell mobo to fit.
Any chance you have the budget for a mostly new rig? I really don't think throwing money into a P4 DC is a great idea.